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  • The Effect of Static Converters on Field Grading Materials in Rotating Machines


    Figure 1. Label in 24pt Arial.


    Silicon Carbide Microvaristors

    Some of objectives


    Silicon carbide microvaristors are the most common type of stress grading materials being used for the design of coils and bars. the main objective of this project is determination of the microvaristor’s response over a wide range of electric field, frequency and temperature. Unfortunately, in spite of previous research in this area, an accurate measurement method, which clears the performance of these materials in mentioned conditions, has not been addressed effectively, owing to problems arising during practical measurement such as thermal run away and response determination in high frequency. On the other hand, such types of insulations are distinctively different from conventional insulations because of Maxwell-Wagner polarisation process, originating from composite mixture of the microvaristors and matrix of insulation. This project aims to provide a viable solution for above addressed issues.

    Two major challenges in this project are the nonlinearity and transient response of field grading materials. The transient response of linear insulation layers (including strand, mica and slot corona protection layer) can be found by frequency domain analysis and implementation of fast Fourier transform for data conversion to the time domain . However, for the nonlinear regime of insulation materials, such correlations are usually not available requiring consideration of permittivity harmonics as well. Furthermore, in order to obtain the fast response of stress grading layer, the responses of the other insulation layers are also required.

    With the introduction of transformer-less converters and their implementation in hydro- power generator systems, electric field distribution of insulation systems is drastically changed. Concerning consequential issues of converters, there are two important agents, namely voltage and temperature according to IEC standards for Type II insulation systems. These effects are summarized in Table I.

    Reza Sargazi Norwegian University of Science and Technology (NTNU) 5/27/2019

    This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 764011.

    Insulation component

    Fundemental frequency

    Impulse repetition frequency

    peak to peak voltage of

    fundemental frequency

    peak to peak voltage of two consecutive


    Impulse rise time

    Turn to turn insulation

    Mainwall insulation


    Corona (CAT) and stress

    grading Layer (SG)

    Note: High Importance Low Importance

    Figure 1. insulation system in Roebel bars

    Voltage Current

    Voltage source

    Series Resistor

    Sample V

    HF and HV Transformer

    Amplifier V

    Matching Impedance

    Stator core




    Traditionally, before the introduction of microvaristors, slot corona protection layer was used both in the slot as well as the end winding area for rotating machines above 4kV. With The advent of microvaristors in the 1970s, manufacturers could cut the corona protection layer just a few centimeters outside the slot without extension into the endwinding region. The resulting local electric field enhancment at this point was also reduced by introducing stress grading layer. this layer has a electric field dependent resistance reducing the high fields and pushing it out into the lower field areas. However, under impulse voltage stresses, its field controlling ability is influenced by factors summerised in table I. The insulation system of Roebel bars for rotating machines is shown in figure 1.

    Table I. different types of stress on insulation system fed by static converters

    Sample AC U I

    Voltage measurement Current measurement

    The effect of pressure finger on performance of SG

    The effect of mutual capacitance of bars on performacne of SG

    The effect of overlapp methods and resin insulation on SG layer

    Dielectric spectroscopy

    Dielectric spectroscopy of insulation is an effective method for insulation modeling. This method can be implemented both in time and frequency domain with possibility of data conversion between both domains in terms of linear dielectric materials. Generally, conductivity of dielectrics can be expressed as following equation.

    𝐽𝐽 𝑡𝑡 = 𝜎𝜎0𝐸𝐸 𝑡𝑡 + 𝜕𝜕

    𝜕𝜕(𝑡𝑡) (𝜀𝜀0 𝐸𝐸(𝑡𝑡) + 𝑃𝑃 𝑡𝑡 )

    Where E(t) is applied electric field, 𝜎𝜎0 is DC conductivity and P(t) is polarization process. General method for frequency domain spectroscopy is shown in below hierarchy.

    Slide Number 1